Process for preparing surface-reacted calcium carbonate and its use

ABSTRACT

The present invention relates to an economic process for preparing surface-reacted calcium carbonate. The present invention further relates to a surface-reacted calcium carbonate having an increased specific BET surface area, and the use of a process for adjusting the specific BET surface area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 13/378,147, filed Jan.9, 2012, which is a U.S. national phase of PCT Application No.PCT/IB2010/052667, filed Jun. 15, 2010, which claims priority toEuropean Application No. 09162738.0, filed Jun. 15, 2009 and U.S.Provisional Application No. 61/269,242, filed Jun. 22, 2009, thecontents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an economic process for preparingsurface-reacted calcium carbonate. The present invention further relatesto a surface-reacted calcium carbonate having an increased BET surfacearea, and the use of a process for adjusting the BET surface area.

BACKGROUND OF THE INVENTION

In practice, calcium carbonate is used in huge quantities in the paper,paint, rubber and plastics industries for various purposes such ascoatings, fillers, extenders and pigments for papermaking as well asaqueous lacquers and paints and in water treatment, and notably as ameans of removing inorganic materials such as heavy metals and/orpharmaceutical waste such as polycyclic compounds, cholesterol and/orendocrine disrupting compounds (EDC).

With respect to precluding aggregation of calcium carbonate particlesand enhancing the affinity of these particles with a substance to whichthe particles are being added, for example as a filler or flocculatingagent, the physical and chemical properties of the surfaces of suchcalcium carbonate particles are amended by treating the calciumcarbonate with fatty acid or a sodium salt of a fatty acid, resin acidor other acids.

In the art, several approaches for improving the chemical and physicalproperties of calcium carbonate have been proposed. For example, U.S.Pat. No. 4,219,590 describes a method for improving calcium carbonate bycausing calcium carbonate particles to undergo contact reaction with anacid gas capable of reacting with calcium carbonate for finelyuniformizing the calcium carbonate particle size and, at the same time,coating the surface of the calcium carbonate particles with the calciumsalt of the acid of the acid gas. U.S. Pat. No. 6,666,953 B1 relates toa pigment, filler or mineral containing a natural calcium carbonate,treated with one of more providers of H₃O⁺ ions and gaseous CO₂,allowing a reduction in the weight of paper for a constant surface areawithout loss of physical properties when it is used as a pigment orcoating filler for said paper. WO 99/02608 A1 describes a method ofproducing a high solids slurry of an acid-resistant precipitated calciumcarbonate, wherein a solids slurry is treated with a chemical additive,such as sodium aluminate, in order to impart acid resistance to thecalcium carbonate.

Additionally, U.S. Pat. No. 5,584,923, U.S. Pat. No. 5,647,902, U.S.Pat. No. 5,711,799, WO 97/08247 A1 and WO 98/20079 A1, respectively,describe calcium carbonate which is acid-resistant to enable its use asa filler material in the making of neutral to weakly acidic paper, and aprocess for producing this acid-resistant calcium carbonate.

Furthermore, WO 2005/121257 A2 discloses a method for producing a drymineral pigment characterized in that it contains a product formed insitu by the multiple reaction of a calcium carbonate and with thereaction product or products of said carbonate with gaseous CO₂ formedin situ and/or from an external supply; and with at least one compoundof formula R—X. WO 2004/083316 A1 refers to mineral pigments containinga product formed in situ by the double and/or multiple reaction betweena calcium carbonate and the reaction product or products of the saidcarbonate with one or more moderately strong to strong H₃O⁺ ion donorsand the reaction product or products of the said carbonate with gaseousCO₂ formed in situ and/or coming from an external supply, and at leastone aluminium silicate and/or at least one synthetic silica and/or atleast one calcium silicate and/or at least one silicate of a monovalentsalt such as sodium silicate and/or potassium silicate and/or lithiumsilicate, preferably such as sodium silicate and/or at least onealuminium hydroxide and/or at least one sodium and/or potassiumaluminate, used in papermaking applications, such as mass filling and/orpaper coating.

U.S. Pat. No. 5,043,017 relates to calcium carbonate, acid-stabilized bythe addition to finely divided calcium carbonate of one of acalcium-chelating agent and a conjugate base, such as sodiumhexametaphosphate, followed by the addition of an acid, such asphosphoric acid.

However, the prior art seems to be silent on economic methods forpreparing and controlling high surface area materials and methods forcontrolling the particle size of materials allowing for the provision ofspecific calcium carbonate materials for the desired purpose. Inparticular, methods for preparing high surface area materials of theprior art require the use of high amounts and concentrations ofmedium-strong to strong acids (having a pK_(a) of less than or equal to2.5) to obtain a desired material having a high specific BET surfacearea. Additionally, the use of such medium-strong to strong acidsnecessitates the use of high safety requirements in order to minimizethe danger to employees. Furthermore, the use of high amounts andconcentrations of said medium-strong to strong acids also result in ahigh and cost-intensive consumption of chemicals and water.

Thus, it would be desirable to have a process available which allows forpreparing surface-reacted calcium carbonate and provides the possibilityof controlling particular parameters such as the specific BET surfacearea and particle size of the calcium carbonate.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide aneconomic process for preparing a surface-reacted calcium carbonate, inwhich the specific BET surface area of the calcium carbonate can becontrolled or adjusted. A further objective of the present invention isthe provision of a process for preparing calcium carbonate, in which thespecific BET surface area of the calcium carbonate particles isincreased. Another objective of the present invention is to provide aprocess, in which the required amount of medium-strong to strong acidsis reduced for a given specific BET surface area of a calcium carbonaterelative to a material prepared by a process of the prior artimplementing only medium-strong to strong acids. Even a furtherobjective of the present invention is to provide a process, in which thespecific BET surface area of a calcium carbonate is increased for agiven amount of medium-strong to strong acid relative to a materialprepared by a process of the prior art implementing only medium-strongto strong acids. A further objective of the present invention is toprovide a process, in which the particle size of the calcium carbonateparticles can be controlled or adjusted. Another objective of thepresent invention is to provide a process, in which the surface-reactedcalcium carbonate can be prepared in high yield.

The present invention aims at solving these and other objectives byproviding a process for preparing surface-reacted calcium carbonate asdescribed in the present invention and defined in the claims.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the present application a process forpreparing a surface-reacted calcium carbonate in an aqueous environmenthas been developed, wherein the process comprises the following steps:

-   -   a) providing a calcium carbonate;    -   b) providing 5-50 wt.-%, based on the weight of calcium        carbonate, of at least one acid having a pK_(a) of less than or        equal to 2.5, wherein the corresponding acid anion is capable of        forming water-insoluble calcium salts;    -   c) providing gaseous CO₂;    -   d) providing at least one water soluble non-polymeric organic        and/or inorganic weak acid and/or a hydrogen salt of said at        least one water soluble non-polymeric organic and/or inorganic        weak acid; and    -   e) contacting said calcium carbonate with said at least one acid        having a pK_(a) of less than or equal to 2.5 of step b), with        said gaseous CO₂ of step c) and with said soluble non-polymeric        organic and/or inorganic weak acid and/or hydrogen salt of said        at least one water soluble non-polymeric organic and/or        inorganic weak acid, of step d);    -   wherein the at least one water soluble non-polymeric organic        and/or inorganic weak acid has a pK_(a) of greater than 2.5 and        wherein its corresponding acid anion is capable of forming        water-insoluble calcium salts.

The inventors surprisingly found that control of particular parametersduring the preparation process defined hereafter, and notably control ofthe moment of addition and quantity of a weak acid to the reactionsystem is key to the formation of a surface-reacted calcium carbonatehaving specific properties, e.g. an increased specific BET surface areaand/or particle size relative to a process of the prior art implementingonly medium-strong to strong acids.

For the purpose of the present application, “water-insoluble calciumsalts” are defined as materials which, when mixed with deionised waterand filtered through a filter having a pore size of 0.2 μm. at 20° C. torecover the liquid filtrate, provide less than or equal to 0.1 g ofrecovered solid material following evaporation at 95 to 100° C. of 100 gof said liquid filtrate. “Soluble (or solubilised) materials” aredefined as materials leading to the recovery of greater than 0.1 g ofrecovered solid material following evaporation at 95 to 100° C. of 100 gof said liquid filtrate.

In accordance with the present invention, an “acid” is defined as aBronsted-Lowry acid, that is to say, it is an H₃O⁺-ion provider. An“acid anion” is defined as a deprotonated form of a Bronsted-Lowry acid,i.e. it is a conjugated base of an acid. An “acid salt” is defined as anH₃O⁺-ion provider that is at least partially neutralised by anon-hydrogen electropositive element. A “salt” is defined as anelectrically neutral ionic compound formed of anions and non-hydrogencations. A “salt” may comprise the anhydrous form as well as formscomprising crystal water (hydrate). A “partially crystalline salt” isdefined as a salt that, on XRD analysis, presents an essentiallydiscrete diffraction diagram.

A “non-polymeric” organic and/or inorganic weak acid for the purpose ofthe present application is defined as an inorganic or organic compoundhaving less than ten repeating structural units connected by covalentchemical bonds. A “hydrogen salt” of said at least one water solublenon-polymeric organic and/or inorganic weak acid is defined as anelectrically neutral ionic compound formed of the acid anion of the atleast one water soluble non-polymeric organic and/or inorganic weak acidand cations, wherein the salt contains at least one hydrogen atom.

A “slurry” in the meaning of the present invention is a suspension(comprising insoluble solids and water and optionally further additives)and usually contains large amounts of solids and is more viscous andgenerally of higher density than the liquid without solids from which itis formed.

In the meaning of the present invention, a “surface-reacted calciumcarbonate” is a material comprising calcium carbonate and insoluble,preferably at least partially crystalline, calcium salts of anions ofsaid acids of steps b) and d) above. In a preferred embodiment, theinsoluble calcium salt extends from the surface of at least part of thecalcium carbonate. The calcium ions forming said at least partiallycrystalline calcium salt of said anion originate largely from thestarting calcium carbonate material.

A “specific BET surface area” (SSA) in the meaning of the presentinvention relates to the specific surface area measured via the methodprovided in the examples section hereafter.

In accordance with the present invention, the calcium carbonatecomprises generally ground (or natural) calcium carbonate (GCC) and/orprecipitated calcium carbonate (PCC), which is also known as syntheticcalcium carbonate.

“Ground calcium carbonate” in the meaning of the present invention is acalcium carbonate obtained from natural sources, marble, chalk orlimestone, and processed through a treatment such as grinding, screeningand/or fractionizing by wet and/or dry, for example by a cyclone.

“Precipitated calcium carbonate” in the meaning of the present inventionis a synthesized material, generally obtained by precipitation followingreaction of carbon dioxide and lime in an aqueous environment or byprecipitation of a calcium and carbonate source in water. Additionally,PCC can also be the product of introducing calcium and carbonate salts,calcium chloride and sodium carbonate for example, in an aqueousenvironment.

The present invention also refers to a surface-reacted calcium carbonatehaving a specific BET surface area (SSA), measured via the methodprovided in the examples section hereafter, of more than 50 m²/g, morepreferably of more than 60 m²/g and most preferably of more than 80m²/g.

Furthermore, the present invention refers to the use of the inventiveprocess for controlling or adjusting properties and parameters ofsurface-reacted calcium carbonate, like the specific BET surface area.

According to one preferred embodiment of the inventive process, the atleast one acid having a pK_(a) of less than or equal to 2.5 is selectedfrom the group consisting of strong acids having a pK_(a) value of lessthan or equal to zero at 25° C., and medium-strong acids having a pK_(a)value of between zero and 2.5, inclusive, at 25° C.

According to another preferred embodiment of the inventive process, themedium-strong acid is selected from the group consisting of H₃PO₄,oxalic acid and mixtures thereof.

According to yet another preferred embodiment of the inventive process,the at least one acid having a pK_(a) of less than or equal to 2.5 isH₃PO₄, preferably H₃PO₄ in a concentration of 20% to 40% (v/v).

According to one preferred embodiment of the inventive process, the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid has a molecular weight of less than 1000 g/mol, preferably less than 750 g/mol and more preferably less than500 g/mol.

According to another preferred embodiment of the inventive process, saidat least one water soluble non-polymeric organic and/or inorganic weakacid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid is selected from thegroup consisting of boric acid, citric acid, sodium dihydrogen citrate,potassium dihydrogen citrate, disodium hydrogen citrate, dipotassiumhydrogen citrate, sodium dihydrogen borate, potassium dihydrogen borate,disodium hydrogen borate, dipotassium hydrogen borate and mixturesthereof.

According to yet another preferred embodiment of the inventive process,said at least one water soluble non-polymeric organic and/or inorganicweak acid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid is added in an amountin the range of 0.1 wt.-% and 20 wt.-%, preferably between 1 wt.-% and15 wt.-%, more preferably between 1 wt.-% and 10 wt.-% and mostpreferably between 1 wt.-% and 5 wt.-%, based on the weight of calciumcarbonate.

According to one preferred embodiment of the inventive process, thecalcium carbonate is first contacted with the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid and then with the at least one acid having a pK_(a) of lessthan or equal to 2.5.

According to another preferred embodiment of the inventive process, thecalcium carbonate is simultaneously contacted with the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid and the at least one acid having a pK_(a) ofless than or equal to 2.5.

According to yet another preferred embodiment of the inventive process,the calcium carbonate is first contacted with the at least one acidhaving a pK_(a) of less than or equal to 2.5 and during its addition,the calcium carbonate is also contacted with the at least one watersoluble non-polymeric organic and/or inorganic weak acid and/or hydrogensalt of said at least one water soluble non-polymeric organic and/orinorganic weak acid.

According to one preferred embodiment of the inventive process, thecalcium carbonate is first contacted with the at least one acid having apK_(a) of less than or equal to 2.5 and after 5% of the total amount ofsaid at least one acid having a pK_(a) of less than or equal to 2.5 havebeen added, the calcium carbonate is contacted with the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid.

According to another preferred embodiment of the inventive process, thecalcium carbonate is first contacted with the at least one acid having apK_(a) of less than or equal to 2.5 and after 10% of the total amount ofsaid at least one acid having a pK_(a) of less or equal to than 2.5 havebeen added, the calcium carbonate is contacted with the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid.

According to yet another preferred embodiment of the inventive process,the calcium carbonate is first contacted with the at least one acidhaving a pK_(a) of less than or equal to 2.5 and after 20% of the totalamount of said at least one acid having a pK_(a) of less than or equalto 2.5 have been added, the calcium carbonate is contacted with the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid.

According to one preferred embodiment of the inventive process, thecalcium carbonate is first contacted with the at least one acid having apK_(a) of less than or equal to 2.5 and after 30% of the total amount ofsaid at least one acid having a pK_(a) of less than or equal to 2.5 havebeen added, the calcium carbonate is contacted with the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid.

According to another preferred embodiment of the inventive process, thecalcium carbonate is first contacted with the at least one acid having apK_(a) of less or equal to than 2.5 and after 50% of the total amount ofsaid at least one acid having a pK_(a) of less than or equal to 2.5 havebeen added, the calcium carbonate is contacted with the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid.

According to yet another preferred embodiment of the inventive process,the calcium carbonate is first contacted with the at least one acidhaving a pK_(a) of less or equal to than 2.5 and after 80% of the totalamount of said at least one acid having a pK_(a) of less than or equalto 2.5 have been added, the calcium carbonate is contacted with the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid.

According to another preferred embodiment of the inventive process, theat least one acid having a pK_(a) of less than or equal to 2.5 isprovided in the form of at least one H₃O⁺-ion provider and 5 wt.-% to 50wt.-%, based on the weight of calcium carbonate, of at least one acidanion, wherein the corresponding acid of the at least one acid anion hasa pK_(a) of less than or equal to 2.5 and said acid anion is capable offorming water-insoluble calcium salts.

According to one preferred embodiment of the inventive process, thecalcium carbonate is further contacted with a compound selected from thegroup consisting of a sodium silicate, a potassium silicate, a lithiumsilicate, an aluminium silicate, a synthetic silica, a calcium silicate,an aluminium hydroxide, a sodium aluminate, a potassium aluminate andmixtures thereof.

According to another preferred embodiment of the inventive process, thesurface reacted calcium carbonate obtained by the inventive process hasa BET specific surface area, measured according to the measurementmethod provided in the examples section hereafter, of at least 20 m²/g,preferably of at least 30 m²/g, more preferably of at least 40 m²/g,even more preferably of at least 50 m²/g, still more preferably of atleast 60 m²/g, yet more preferably of at least 70 m²/g and mostpreferably of at least 80 m²/g.

According to one preferred embodiment of the present invention, theproduct obtained by the inventive process, i.e. the aqueous suspensionis dried after step e) in order to preferably yield a dry or solidproduct.

In the following, the steps a) to e) of the inventive method forpreparing surface-reacted calcium carbonate are described in moredetail:

Step a): Providing Calcium Carbonate Comprising Aqueous Preparation

According to step a) of the process of the present invention, a calciumcarbonate is provided.

Preferably, the calcium carbonate is selected from ground (or natural)calcium carbonate (GCC) and/or precipitated calcium carbonate (PCC),which is also known as synthetic calcium carbonate.

GCC is understood to be a naturally occurring form of calcium carbonate,mined from sedimentary rocks such as limestone or chalk, or frommetamorphic marble rocks. GCC is almost exclusively of the calciticpolymorph, which is said to be trigonal-rhombohedral and represents themost stable of the calcium carbonate polymorphs.

Preferably, the natural calcium carbonate is selected from the groupcomprising marble, chalk, calcite, dolomite, limestone and mixturesthereof.

By contrast, calcium carbonate polymorphs of the PCC type often include,in addition to calcites, less stable polymorphs of the aragonitic-type,which has an orthorhombic, acicular crystal shape, and hexagonalvateritic-type, which has an even lower stability than aragonite. Thedifferent PCC forms may be identified according to their characteristicx-ray powder diffraction (XRD) peaks. PCC synthesis most commonly occursby a synthetic precipitation reaction that includes a step of contactingcarbon dioxide with a solution of calcium hydroxide, the latter beingmost often provided on forming an aqueous suspension of calcium oxide,also known as burnt lime, and the suspension of which is commonly knownas milk of lime. Depending on the reaction conditions, this PCC canappear in various forms, including both stable and unstable polymorphs.Indeed, PCC often represents a thermodynamically unstable calciumcarbonate material.

When referred to in the context of the present invention, PCC shall beunderstood to mean synthetic calcium carbonate products obtained bycarbonation of a slurry of calcium hydroxide, commonly referred to inthe art as a slurry of lime or milk of lime when derived from finelydivided calcium oxide particles in water.

Preferred synthetic calcium carbonate is precipitated calcium carbonatecomprising aragonitic, vateritic or calcitic mineralogical crystal formsor mixtures thereof.

In a preferred embodiment, the calcium carbonate is ground prior to theprocess for preparing surface-reacted calcium carbonate. The grindingstep can be carried out with any conventional grinding device such as agrinding mill known to the skilled person.

In a preferred embodiment, the calcium carbonate of step a) has a weightmedium diameter of 0.01 μm to 10 μm, and more preferably of 0.5 μm to 2μm, as measured according to the measurement method provided in theExamples section here below.

In a preferred process for the preparation of the aqueous suspension,the calcium carbonate, either finely divided (such as by grinding) ornot, is suspended in water and thus in the form of a slurry.

In this preferred embodiment, said slurry preferably has a pH of lessthan 11, preferably of less than 10.5, as measured according to themeasurement method described in the Examples section here below.

Preferably, the aqueous calcium carbonate slurry has a solids contentgreater than or equal to 10 wt.-%, and more preferably of between 10wt.-% and 80 wt.-%, based on the weight of the slurry. The applicantwould remark that in the case of very high solids contents, it is arequirement to have sufficient water for the reactions following thecontacting of step e) to occur. More preferably, the aqueous calciumcarbonate slurry has a solid content in the range of 16 wt.-% to 60wt.-%, and even more preferably in the range of 16 wt.-% to 40 wt.-%,based on the weight of the slurry.

Step b): Providing at Least One Acid Having a pK_(a) of Less than orEqual to 2.5

According to step b) of the process of the present invention, 5 wt.-% to50 wt.-%, based on the weight of calcium carbonate, of at least one acidhaving a pK_(a) of less than or equal to 2.5 is provided, wherein thecorresponding acid anion is capable of forming water-insoluble calciumsalts. Said insoluble corresponding calcium salts may, in addition tosaid acid anion, include OH− ions and/or crystal water.

The H₃O⁺-ions provided by the at least one acid having a pKa of lessthan or equal to 2.5 serve to partially dissolve the calcium carbonateparticle, generating calcium ions for subsequent precipitation of aninsoluble, at least partially crystalline calcium salt of the anion atthe calcium carbonate surface.

Preferably, the acid added to the aqueous suspension containing thecalcium carbonate has a pK_(a) at 25° C. of 2.5 or less. Formedium-strong acids such as H₃PO₄, oxalic acid or mixtures thereof, apK_(a) at 25° C. from 0 to 2.5 may be observed. These one or moremedium-strong to strong acids can be added to the suspension as aconcentrated solution or diluted solution.

In one preferred embodiment of the present invention, H₃PO₄ is used asthe at least one acid having a pK_(a) of less than or equal to 2.5. Itis especially preferred to use H₃PO₄ in a concentration of at least 10%(v/v), more preferably of at least 20% (v/v) and most preferably of atleast 30% (v/v). In another preferred embodiment, it is preferred to useH₃PO₄ in a concentration of 20% to 40% (v/v).

In one preferred embodiment, the at least one acid having a pK_(a) ofless than or equal to 2.5 is provided in the form of:

-   -   (i) at least one H₃O⁺-ion provider having a pKa of less than or        equal to 2.5, wherein the corresponding acid anion is capable of        forming a water-soluble calcium salt, and    -   (ii) at least one anion, provided in the form of a water-soluble        salt or hydrogen salt, wherein the corresponding acid of this at        least one anion has a pK_(a) of less than or equal to 2.5 and        wherein said anion is capable of forming water-insoluble calcium        salts.

Said at least one H₃O⁺-ion provider of (i) is preferably selected fromthe group consisting of nitric acid, sulphuric acid, hydrochloric acid,H₂SO₃, HSO₄ ⁻═ or mixtures thereof.

In an especially preferred embodiment, said at least one anion of (ii)is added in a quantity corresponding to a quantity to between 5 wt.-%and 50 wt.-%, based on the weight of calcium carbonate.

In this case, said at least one H₃O⁺-ion provider and the at least oneanion are preferably provided separately, preferably such that the atleast one H₃O⁺-ion provider of step (i) is contacted with calciumcarbonate before and/or during the contacting of calcium carbonate withthe anion of (ii).

The cationic group of the water-soluble salt or hydrogen salt of (ii) ispreferably selected from the group comprising lithium, sodium,potassium, rubidium and mixtures thereof. In one preferred embodiment,the salt of said at least one anion of (ii) is selected from the groupcomprising sodium sulphate (Na₂SO₄) sodium hydrogen sulphate (NaHSO₄),sodium sulphite (Na₂SO₃), sodium phosphate (Na₃PO₄), disodium hydrogenphosphate (Na₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄), disodiumoxalate (Na₂C₂O₄), sodium hydrogen oxalate (NaHC₂O₄) and mixturesthereof. Preferably, the salt of the at least one acid anion is a sodiumphosphate (Na_(3-x)H_(x)PO₄, where x=2, 1 or 0). Such salt in themeaning of the present invention shall be understood to comprise saltsin the anhydrous form as well as forms comprising crystal water(hydrate).

The at least one anion in the form of a salt of (ii) can be added to theaqueous suspension comprising calcium carbonate in any appropriate solidform, e.g. in the form of granules or a powder. Alternatively oradditionally, at least one anion in the form of a salt can be added tothe aqueous suspension comprising calcium carbonate in the form of aconcentrated solution or a more diluted solution.

In one preferred embodiment of the present invention, the at least oneacid having a pK_(a) of less than or equal to 2.5 is added continuouslyat essentially the same rate (amount/time) over a period of 2 h or less,preferably over a period of 1.5 h or less, more preferably over a periodof 1 h or less and most preferably over a period of 30 min or less tothe aqueous suspension comprising calcium carbonate. In an especiallypreferred embodiment the at least one acid having a pK_(a) of less thanor equal to 2.5 is added over a period of 15 min or less to the aqueoussuspension comprising the calcium carbonate.

In another preferred embodiment the at least one acid having a pK_(a) ofless than or equal to 2.5 is added in one portion to the aqueoussuspension comprising the calcium carbonate.

Said acid of step b) may be added in the form of a soluble neutral oracid salt, or in the form of an acid, provided it is solubilised beforeand/or during step e).

In a preferred embodiment, said at least one acid having a pK_(a) ofless than or equal to 2.5 is added in a quantity corresponding tobetween 5% and 50% by weight, preferably between 10% and 30% by weight,based on the weight of said calcium carbonate.

Following the addition of the at least one acid having a pK_(a) of lessthan or equal to 2.5 to the slurry, the pH of the slurry, as measuredaccording to the measurement method given in the Examples section herebelow, may drop temporarily to a value below 6.0.

Step c): Providing Gaseous CO₂

According to step c) of the process of the present invention, gaseousCO₂ is provided.

On contacting calcium carbonate with an acid having a pK_(a) of lessthan or equal to 2.5, the required carbon dioxide may be formed in situfrom the carbonate. Alternatively or additionally, the gaseous carbondioxide can be supplied from an external source.

Acid treatment and treatment with gaseous carbon dioxide can be carriedout simultaneously and automatically takes place when a strong acid isused. It is also possible to carry out an inventive acid treatmentfirst, e.g. with a medium strong acid having a pK_(a) in the range of 0to 2.5 such as H₂SO₃, HSO₄ ⁻, H₃PO₄, oxalic acid, followed by treatmentwith gaseous carbon dioxide supplied from an external source.

In the case where gaseous carbon dioxide is provided, the concentrationof gaseous carbon dioxide in the aqueous suspension throughout step e)is, in terms of volume, such that the ratio (volume ofsuspension):(volume of gaseous CO₂) is from 1:0.05 to 1:20, even morepreferably 1:0.05 to 1:5. In another preferred embodiment, said ratio of(volume of suspension):(volume of gaseous CO₂) is from 1:0.05 to 1:20,even more preferably 1:0.05 to 1:5 is maintained during the process ofthe present invention.

Step d): Providing at Least One Acid Having a pK_(a) of Greater than 2.5

According to step d) of the process of the present invention, at leastone water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid is provided, wherein said weak acidhas a pK_(a) of greater than 2.5 and wherein the corresponding acidanion is capable of forming water-insoluble calcium salts.

The at least one water soluble non-polymeric organic and/or inorganicweak acid added to the aqueous suspension comprising the calciumcarbonate has a pK_(a) at 25° C. of 2.5 or more. Preferably, said weakacid added to the aqueous suspension containing the calcium carbonatehas a pK_(a) at 25° C. of 3.0 or more. The one or more solublenon-polymeric organic and/or inorganic weak acids can be added to theaqueous suspension as a concentrated solution or a diluted solution.

In a preferred embodiment, the at least one water soluble non-polymericorganic and/or inorganic weak acid has a molecular weight of less than 1000 g/mol, preferably less than 750 g/mol, more preferably less than 500g/mol.

The at least one water soluble non-polymeric organic and/or inorganicweak acid added to the aqueous suspension is preferably selected fromthe group consisting of citric acid, boric acid, or mixtures thereof.

Alternatively or additionally, the at least one water solublenon-polymeric organic and/or inorganic weak acid may be added to theaqueous suspension in the form of a hydrogen salt. A “hydrogen salt” inthe meaning of the present application is defined as an electricallyneutral ionic compound formed of the acid anion of the at least onewater soluble non-polymeric organic and/or inorganic weak acid andcations, wherein the salt contains at least one hydrogen atom. Thecationic group of such salt is preferably selected from an alkali metal,preferably from lithium, sodium, potassium, rubidium and mixturesthereof.

In one preferred embodiment, the hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid isselected from the group consisting of sodium and/or potassium and/orlithium dihydrogen citrate, disodium and/or dipotassium and/or dilithiumhydrogen citrate, sodium and/or potassium and/or lithium dihydrogenborate, disodium and/or dipotassium and/or dilithium borate and mixturesthereof. Preferably, the salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid is disodium hydrogencitrate (Na₂C₆H₆O₇).

In a preferred embodiment, said at least one water soluble non-polymericorganic and/or inorganic weak acid and/or hydrogen salt of said at leastone water soluble non-polymeric organic and/or inorganic weak acid isadded in a quantity corresponding to between 0.1 wt.-% and 20 wt.-%,preferably between 1 wt.-% and 15 wt.-%, more preferably between 1.5wt.-% and 10 wt.-% and most preferably between 2 wt.-% and 5 wt.-%,based on the weight of said calcium carbonate.

Preferably, the weight ratio of the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid to the at least one acid having a pK_(a) of less than or equalto 2.5 in the suspension is from 1:100 to 100:1, more preferably 1:75 to75:1, even more preferably 1:50 to 50:1, still more preferably 1:50 to1:1, and most preferably 1:12 to 1:3.

Subsequent to the carbon dioxide treatment and the treatment with atleast one acid having a pK_(a) of less than or equal to 2.5 and at leastone water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid, the pH of the aqueous suspension,measured at 20° C., usually may reach a value of greater than 6.0,preferably greater than 6.5, more preferably greater than 7.0, even morepreferably greater than 7.5. In other words, a surface-reacted calciumcarbonate as an aqueous suspension having a pH of greater than 6.0,preferably greater than 6.5, more preferably greater than 7.0, even morepreferably greater than 7.5 is obtained. If the aqueous suspension isallowed to reach equilibrium, the pH usually is greater than 7. A pH ofgreater than 6.0 may be adjusted without the addition of a base whenstirring of the aqueous suspension is continued for a sufficient timeperiod, preferably 1 hour to 10 hours, more preferably 1 to 5 hours.

Alternatively, prior to reaching an equilibrium state, which occurs at apH greater than 7, the pH of the aqueous suspension may be increased toa value greater than 6 by adding a base subsequent to carbon dioxidetreatment. Any conventional base such as sodium hydroxide or potassiumhydroxide can be used.

Step e): Treatment of the Calcium Carbonate

According to step e) of the process of the present invention, thecalcium carbonate is contacted in an aqueous slurry environment with theacid having a pK_(a) of less than or equal to 2.5 of step b) with saidgaseous CO₂ of step c) and with said at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid of step d).

Said calcium carbonate according to the inventive process is contactedor treated with at least one acid having a pK_(a) of less than or equalto 2.5 (provided in step b)) and at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid (provided in step d)) via the following routes:

Route IA: simultaneously contacting said calcium carbonate with the atleast one acid having a pK_(a) of less than or equal to 2.5 of step b)and with the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid of step d);

Route IIA: first contacting said calcium carbonate with the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid of step d) and then with the at least oneacid having a pK_(a) of less than or equal to 2.5 of step b);

Route IIIA: first contacting said calcium carbonate with the at leastone acid having a pK_(a) of less than or equal to 2.5 of step b) and,during its addition, the calcium carbonate is also contacted with the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid of step d).

In case where the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid and theat least one acid having a pK_(a) of less than or equal to 2.5 aresimultaneously added to the calcium carbonate (Route IA), the weak acidand/or its hydrogen salt can be blended or mixed with the at least oneacid having a pK_(a) of less than or equal to 2.5 prior to the addition.

The mixture of acids is then added to the aqueous suspension in oneportion or continuously at essentially the same rate (amount/time) overa period of 2 h or less, preferably over a period of 1.5 h or less, morepreferably over a period of 1 h or less and most preferably over aperiod of 30 min or less and in an especially preferred embodiment overa period of 15 min or less.

By using Route IA of the inventive process, it is possible to obtaincarbonate particles having an increased specific BET surface area. Saidprocess may provide calcium carbonate particles having a specific BETsurface area that is at least 10% greater, more preferably at least 20%greater and most preferably at least 30% greater than the specific BETsurface area obtained by contacting the same calcium carbonate providedin step a) either simultaneously or in distinct steps, with the at leastone acid having a pK_(a) of less than or equal to 2.5 and gaseous CO₂but without adding the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid.

If the at least one water soluble non-polymeric organic and/or inorganicweak acid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid is added to the calciumcarbonate suspension before contacting said calcium carbonate with theat least one acid having a pK_(a) of less than or equal to 2.5 (RouteIIA), the weak acid and/or its hydrogen salt may, for example, be addedto the aqueous suspension in one portion or continuously at essentiallythe same rate (amount/time) over a period of 15 min or less, preferablyover a period of 10 min or less, more preferably over a period of 5 minor less.

After the complete addition of the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid, the at least one acid having a pK_(a) of less than or equalto 2.5 is added to the aqueous suspension in one portion or continuouslyat essentially the same rate (amount/time) over a period of 2 h or less,preferably over a period of 1.5 h or less, more preferably over a periodof 1 h or less and most preferably over a period of 30 min or less andin an especially preferred embodiment over a period of 15 min or less.

By using Route IIA of the inventive process, it is possible to obtaincalcium carbonate particles having a particular high weight mediandiameter. Said process may provide calcium carbonate particles having amedian diameter that is at least 10% greater, more preferably at least20% greater and most preferably at least 50% greater than the mediandiameter obtained by contacting the same calcium carbonate provided viaRoute IA; i.e. by treating said calcium carbonate simultaneously with amixture of at least one acid having a pK_(a) of less than or equal to2.5 and at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid.

All median diameters of surface-reacted calcium carbonate materialsreferred to in the present invention are measured according themeasurement method provided in the examples section hereafter.

Furthermore, by using Route IIA of the inventive process, it is possibleto obtain calcium carbonate particles having an increased specific BETsurface area. Said process via Route IIA may provide calcium carbonateparticles having a specific BET surface area that is at least 10%greater, more preferably at least 20% greater and even more preferablyat least 50% greater than the specific BET surface area obtained bycontacting the same calcium carbonate provided in step a) eithersimultaneously or in distinct steps, with the at least one acid having apK_(a) of less than or equal to 2.5 and gaseous CO₂ but without addingthe at least one water soluble non-polymeric organic and/or inorganicweak acid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid.

If the at least one water soluble non-polymeric organic and/or inorganicweak acid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid is added to the aqueoussuspension during contacting or treating said calcium carbonate with theat least one acid having a pK_(a) of less than or equal to 2.5 (RouteIIIA), the at least one acid having a pK_(a) of less than or equal to2.5 may, for example, be added to the aqueous suspension continuously atessentially the same rate (amount/time) over a period of 2 h or less,preferably over a period of 1.5 h or less, more preferably over a periodof 1 h or less and most preferably over a period of 30 min or less andin an especially preferred embodiment over a period of 15 min or less.

During the addition of the at least one acid having a pK_(a) of lessthan or equal to 2.5, preferably after 5%, after 10%, after 20%, after30%, after 50% or after 80% of the time period required for continuouslyadding said acid(s) having a pK_(a) of less than or equal to 2.5 to thecalcium carbonate slurry, the calcium carbonate may be contacted withthe at least one water soluble non-polymeric organic and/or inorganicweak acid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid. It is to be understoodby the skilled person that, e.g., 5% of the time period necessary forcontinuously adding said acid(s) having a pK_(a) of less than or equalto 2.5 essentially corresponds to the addition of 5% of the total amountof said acid(s) having a pK_(a) of less than or equal to 2.5 to be addedsince the acid according to the present invention preferably is added atthe same rate (amount/time).

Said weak acid and/or its hydrogen salt may for example be added to theaqueous suspension continuously at essentially the same rate(amount/time) over a period of 15 min or less, preferably over a periodof 10 min or less, more preferably over a period of 5 min or less.

When using Route IIIA of the inventive process, the at least one watersoluble non-polymeric organic and/or inorganic weak acid and/or hydrogensalt of said at least one water soluble non-polymeric organic and/orinorganic weak acid may be added near the beginning of the process ofadding said acid(s) having a pK_(a) of less than or equal to 2.5. Forexample, the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid may be addedafter 5% or after 10% of the time period necessary for continuouslyadding said acid(s) having a pK_(a) of less than or equal to 2.5. Byusing route IIIA, it is possible to obtain calcium carbonate particleshaving an increased weight median diameter. Said process may providecalcium carbonate particles having a median diameter that is at least 5%greater, more preferably at least 10% greater and most preferably atleast 20% greater than the median diameter obtained by contacting thesame calcium carbonate provided in step a) with gaseous carbon dioxideand the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid at the end ofthe process of adding said acid(s) having a pK_(a) of less than or equalto 2.5, i.e. for example after 80% of the time period necessary forcontinuously adding said acid(s) having a pK_(a) of less than or equalto 2.5.

Accordingly, it is possible to obtain calcium carbonate particles havinga decreased median diameter by using Route IIIA of the inventiveprocess, if the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid is added nearthe end of the process of adding said acid(s) having a pK_(a) of lessthan or equal to 2.5, for example after 80% of the time period necessaryfor continuously adding said acid(s) having a pK_(a) of less than orequal to 2.5 (and essentially corresponding to 80% of the total amountof said acid(s) having a pK_(a) of less than or equal to 2.5 to beadded).

Furthermore, by using Route IIIA of the inventive process, it is alsopossible to obtain calcium carbonate particles having an increasedspecific BET surface area. The process may provide calcium carbonateparticles having a specific BET surface area that is at least 10%greater, more preferably at least 15% greater and even more preferablyat least 20% greater than the specific BET surface area obtained bycontacting the same calcium carbonate provided in step a) with the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid near the end of the process of addingsaid acid(s) having a pK_(a) of less than or equal to 2.5, i.e., forexample, after 80% of the time period necessary for continuously addingsaid acid(s) having a pK_(a) of less than or equal to 2.5.

Accordingly, it is possible to selectively control or adjust specificparameters or properties of the calcium carbonate like the specific BETsurface area and the median diameter by using Route IIIA of theinventive process. The addition of the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid at the end of the process of adding said acid(s) having apK_(a) of less than or equal to 2.5 to the calcium carbonate dispersionmay lead to calcium carbonate particles having a decreased or lowerspecific BET surface area and a lower weight median diameter, whereasthe addition of the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid at thebeginning of the process of acid(s) having a pK_(a) of less than orequal to 2.5 addition, e.g. after 5 wt.-% of the acid(s) having a pK_(a)of less than or equal to 2.5 have been added, may lead to calciumcarbonate particles having an increased specific BET surface area and ahigher or increased weight median diameter.

In a preferred embodiment, step e) is carried out at temperatures ofabove room temperature, more preferably of above 50° C., and even morepreferably of above 60° C.

In a preferred embodiment, the slurry is mixed so as to develop anessentially laminar flow.

In an optional embodiment, step e) takes place in the presence of atleast one compound selected from the group consisting of a silicate of amonovalent salt such as a sodium silicate, a potassium silicate, alithium silicate, an aluminium silicate, a synthetic silica, a calciumsilicate, an aluminium hydroxide, a sodium aluminate, a potassiumaluminate and mixtures thereof. Preferably the monovalent salt is sodiumsilicate.

According to a further embodiment of the present invention, step e)takes place in the presence of an inert gas, which is bubbled throughthe slurry.

The acid treatment steps and/or the carbon dioxide treatment step may berepeated once or several times, if appropriate.

In a preferred embodiment of the present invention, the obtained slurrycomprising the surface-reacted calcium carbonate has a solid content, asmeasured according to the measurement method described in the Examplessection hereafter, of up to 25% by weight, preferably between 5% and 20%by weight.

In a preferred embodiment, the aqueous phase of the obtainedsurface-reacted calcium carbonate slurry may be replaced with deionisedwater. In a more preferred embodiment, the aqueous phase of saidsurface-reacted calcium carbonate slurry is collected and recirculatedinto the process according to the present invention as a means toprovide all or part of the solubilised calcium ions. This isparticularly of interest when the process according to the invention isa continuous process.

The obtained surface-reacted calcium carbonate slurry may beconcentrated, optionally up to the point of obtaining a drysurface-reacted calcium carbonate product. If the aqueous suspensiondescribed above is dried, the obtained solid (i.e. dry or containing aslittle water that it is not in a fluid form) surface-reacted calciumcarbonate may be in the form of granules or a powder. In the case of adry product, this product may additionally be treated with fatty acids.In the case of a dry product, this product may be additionally washedwith water.

Thus, a slurry of surface-reacted calcium carbonate is obtained, whereinsaid surface-reacted calcium carbonate comprises an insoluble, at leastpartially crystalline calcium salt of an anion derived from the at leastone acid having a pK_(a) of less than or equal to 2.5 and/or the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid, which preferably extends from thesurface of at least part of the calcium carbonate, provided in step a).

This surface-reacted calcium carbonate provides a specific BET surfacearea that is at least 5% greater than the BET specific surface areaobtained following contacting the same calcium carbonate provided instep a), either simultaneously or in distinct steps, with the at leastone acid having a pK_(a) of less than or equal to 2.5 and gaseous CO₂but without adding the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid.

In a preferred embodiment the resulting surface-reacted calciumcarbonate has a specific BET surface area that is at least 10% greater,more preferably at least 20% greater and most preferably at least 50%greater than the specific BET surface area of a calcium carbonateobtained by contacting the same calcium carbonate either simultaneouslyor in distinct steps, with the at least one acid having a pK_(a) of lessthan or equal to 2.5 and gaseous CO₂ but without adding the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid.

The surface-reacted calcium carbonate obtained by the inventive processmay further provide a specific BET surface area that is the same as thespecific BET surface area obtained by contacting the same calciumcarbonate either simultaneously or in distinct steps, with the at leastone acid having a pK_(a) of less than or equal to 2.5 and gaseous CO₂but without adding the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid. In thiscase, the required amount of the at least one acid having a pK_(a) ofless or equal to than 2.5 is reduced of at least 5 wt.-%, morepreferably of at least 10 wt.-%, even more preferably of at least 15wt-% and most preferably of at least 20 wt.-%, based on the weight ofsaid at least one acid having a pK_(a) of less than or equal to 2.5.

In a preferred embodiment, the surface-reacted calcium carbonateobtained by the inventive process has a specific surface area of morethan 20 m²/g, e.g. 20 m²/g to 200 m²/g, preferably more than 30 m²/g,e.g. 30 m²/g to 150 m²/g, more preferably more than 40 m²/g, even morepreferably more than 50 m²/g, still more preferably more than 60 m²/g,yet more preferably more than 70 m²/g and most preferably more than 80m²/g, measured using nitrogen and the BET method according to the methodprovided in the examples section hereafter.

Furthermore, it is preferred that the surface-reacted natural orsynthetic calcium carbonate has a weight median grain diameter of from0.1 μm to 50 μm, preferably of from 1 μm to 25 μm, more preferably offrom 3 μm to 15 μm, and most preferably of from 5 μm to 12 μm asmeasured according to the measurement method provided in the examplessection hereafter.

In a preferred embodiment, the surface-reacted calcium carbonate has aspecific BET surface area within the range of 20 m²/g to 150 m²/g or 30m²/g to 200 m²/g and a weight median grain diameter within the range of0.1 μm to 50 μm.

The inventive surface-reacted calcium carbonate or a slurry of saidsurface-reacted calcium carbonate may be used (in the form of slurriesor in the form of dry products) in paper, tissue paper, plastics,paints, or as a controlled release or water treatment agent.

The surface-reacted calcium carbonate obtained by the process of thepresent invention is preferably brought into contact with water to bepurified, e.g. industrial waste water, drinking water, urban wastewater, waste water from breweries, or water in the paper industry, byany conventional means known to the skilled person.

The surface-reacted calcium carbonate can be added as an aqueoussuspension, e.g. the suspension described above. Alternatively, it canbe added to the water to be purified in any appropriate solid form, e.g.in the form of granules or a powder or in the form of a cake.

The water may contain organic impurities, e.g. resulting from humanwaste, organic materials, soil, surfactants as well as inorganicimpurities, in particular heavy metal impurities such as iron- ormanganese-containing compounds. Harmful components that can be removedfrom the water with the purification process of the present inventionalso include microorganisms such as bacteria, fungi, archaea, orprotists.

The following examples are meant to illustrate the invention withoutrestricting its scope:

EXAMPLES Measurement Methods

The following measurement methods are used to evaluate the parametersgiven in the examples and claims.

Specific Surface Area (SSA) of a Material

The specific surface area is measured via the BET method according toISO 9277 using nitrogen, following conditioning of the sample by heatingat 250° C. for a period of 30 minutes. Prior to such measurements, thesample is filtered within a Büchner funnel, rinsed with deionised waterand dried overnight at 90 to 100° C. in an oven. Subsequently the drycake is ground thoroughly in a mortar and the resulting powder placed ina moisture balance at 130° C. until a constant weight is reached.

Particle Size Distribution (Mass % Particles with a Diameter <X) andWeight Median Grain Diameter (d₅₀) of Non-Surface Reacted CalciumCarbonate Particulate Material (i.e. Calcium Carbonate StartingMaterial)

Weight median grain diameter and grain diameter mass distribution of aparticulate material, such as calcium carbonate, are determined via thesedimentation method, i.e. an analysis of sedimentation behaviour in agravimetric field. The measurement is made with a Sedigraph™ 5120.

The method and the instrument are known to the skilled person and arecommonly used to determine grain size of fillers and pigments. Themeasurement is carried out in an aqueous solution of 0.1 wt.-% Na₄P₂O₇.The samples were dispersed using a high speed stirrer and ultrasonic.

Median Grain Diameter (d₅₀) of Surface-Reacted Calcium CarbonateMaterial

Median grain diameter of surface-reacted calcium carbonate material isdetermined using a Malvern Mastersizer 2000 Laser Diffraction System.

X-Ray Diffraction (XRD)

Crystallographic structures of materials were identified based on theXRD analytical technique using Brucker AXS:D8 Advance instrumentation,scanning 2 to 70° 2theta at a scanning speed of 0.5 seconds/step and astep size of 0.01° 2theta. Analysis of the resulting spectra was basedon the PDF 2 database of reference spectra issued by the InternationalCentre for Diffraction Data.

pH of an Aqueous Slurry

The pH of the aqueous suspension is measured using a standard pH-meterat approximately 25° C.

Solids Content of an Aqueous Slurry

The slurry solids content (also known as “dry weight”) is determinedusing a Moisture Analyser HR73 commercialised by Mettler-Toledo with thefollowing settings: temperature of 120° C., automatic switch off 3,standard drying, 5-20 g of slurry.

Example 1

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention incomparison to contacting the same calcium carbonate with the at leastone acid having a pK_(a) of less than or equal to 2.5 and gaseous CO₂but without adding the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid andcontacting the same calcium carbonate with the at least one watersoluble non-polymeric organic and/or inorganic weak acid and/or hydrogensalt of said at least one water soluble non-polymeric organic and/orinorganic weak acid and gaseous CO₂ but without adding the at least oneacid having a pK_(a) of less than or equal to 2.5.

1. Preparation of Calcium Carbonate Slurries

a. Calcium Carbonate Slurry V1 (Prior Art)

A calcium carbonate slurry V1 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. Following this addition, the slurryis stirred for an additional 5 minutes by using a propeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

b. Calcium Carbonate Slurry V2

A calcium carbonate slurry V2 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,citric acid in an amount corresponding to 5% by weight on calciumcarbonate weight is added via a peristaltic pump to the calciumcarbonate slurry. After finishing the addition of citric acid, H₃PO₄ inan amount corresponding to 10% by weight on calcium carbonate weight andto approximately 1×10⁻³ moles H₃PO₄ per gram calcium carbonate is addedvia a peristaltic pump to the calcium carbonate slurry over a period of15 minutes. Following this addition, the slurry is stirred for anadditional 5 minutes by using a propeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

c. Calcium Carbonate Slurry V3

A calcium carbonate slurry V3 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,citric acid in an amount corresponding to 5% by weight on calciumcarbonate weight is added via a peristaltic pump to the calciumcarbonate slurry. Following this addition, the slurry is stirred for anadditional 5 minutes by using a propeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 1 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas and short-term absorptionrates:

Sample V1 V2 V3 % H₃PO₄ 10 10 none % citric acid none  5 5 SSA of   24.0  46.0   7.0 product/m²/g

Example 2

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention,wherein the concentration of the at least one acid having a pK_(a) ofless than or equal to 2.5 is varied.

1. Preparation of Calcium Carbonate Slurries

a. Calcium Carbonate Slurry V4

A calcium carbonate slurry V4 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 30% by weight on calcium carbonateweight and to approximately 3×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 15% of elapsed time foraddition of H₃PO₄ (2 min), citric acid in an amount corresponding to 5%by weight on calcium carbonate weight is added via a peristaltic pump tothe calcium carbonate slurry. Following the complete addition of acids,the slurry is stirred for an additional 5 minutes by using a propellertype mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

b. Calcium Carbonate Slurry V5

A calcium carbonate slurry V5 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 20% by weight on calcium carbonateweight and to approximately 2×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 15% of elapsed time foraddition of H₃PO₄ (2 min), citric acid in an amount corresponding to 5%by weight on calcium carbonate weight is added via a peristaltic pump tothe calcium carbonate slurry. Following the complete addition of acids,the slurry is stirred for an additional 5 minutes by using a propellertype mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

c. Calcium Carbonate Slurry V6

A calcium carbonate slurry V6 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 15% of elapsed time foraddition of H₃PO₄ (2 min), citric acid in an amount corresponding to 5%by weight on calcium carbonate weight is added via a peristaltic pump tothe calcium carbonate slurry. Following the complete addition of acids,the slurry is stirred for an additional 5 minutes by using a propellertype mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 2 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas:

Sample V4 V5 V6 % H₃PO₄ 30 20 10 % citric acid 5 5 5 SSA of 108.5 84.948.3 product/m²/g

Example 3

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention,wherein the concentration of the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid is varied.

1. Preparation of Calcium Carbonate Slurry

a. Calcium Carbonate Slurry V7

A calcium carbonate slurry V7 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 50% of elapsed time foraddition of H₃PO₄ (7.5 min), citric acid in an amount corresponding to1% by weight on calcium carbonate weight is added via a peristaltic pumpto the calcium carbonate slurry. Following the complete addition ofacids, the slurry is stirred for an additional 5 minutes by using apropeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

b. Calcium Carbonate Slurry V8

A calcium carbonate slurry V8 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 50% of elapsed time foraddition of H₃PO₄ (7.5 min), citric acid in an amount corresponding to2.5% by weight on calcium carbonate weight is added via a peristalticpump to the calcium carbonate slurry. Following the complete addition ofacids, the slurry is stirred for an additional 5 minutes by using apropeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 3 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas:

Sample V1 V7 V8 % H₃PO₄ 10 10 30 % citric acid none  1   2.5 Time ofaddition —   50%   50% of citric acid relative to the total time ofH₃PO₄ addition SSA of 24   34.5   38.4 product/m²/g

Example 4

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention,wherein the calcium carbonate is contacted with 30% of the at least oneacid having a pK_(a) of less than or equal to 2.5 and the time point ofadding the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid is varied.

1. Preparation of Calcium Carbonate Slurries

a. Calcium Carbonate Slurry V9

A calcium carbonate slurry V9 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,citric acid in an amount corresponding to 5% by weight on calciumcarbonate weight is added via a peristaltic pump to the calciumcarbonate slurry. After the complete addition of citric acid, H₃PO₄ inan amount corresponding to 30% by weight on calcium carbonate weight andto approximately 3×10⁻³ moles H₃PO₄ per gram calcium carbonate is addedvia a peristaltic pump to the calcium carbonate slurry over a period of15 minutes. Following the complete addition of acids, the slurry isstirred for an additional 5 minutes by using a propeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 4 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas:

Sample V4 V9 % H₃PO₄ 30   30 % citric acid 5  5 Time of addition   15%Pre- of citric acid Addition relative to the total time of H₃PO₄addition SSA of  108.5    105.5 product/m²/g d₅₀/μm   8.20    11.94

Example 5

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention,wherein the calcium carbonate is contacted with 10% of the at least oneacid having a pK_(a) of less than or equal to 2.5 and the time point ofadding the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid is varied.

1. Preparation of Calcium Carbonate Slurries

a. Calcium Carbonate Slurry V10

A calcium carbonate slurry V10 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate blended with citric acid in an amount corresponding to 5% byweight on calcium carbonate weight is added via a peristaltic pump tothe calcium carbonate slurry over a period of 15 minutes. Following thecomplete addition of acids, the slurry is stirred for an additional 5minutes by using a propeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

b. Calcium Carbonate Slurry V11

A calcium carbonate slurry V11 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 15% of elapsed time foraddition of H₃PO₄ (2 min), citric acid in an amount corresponding to 5%by weight on calcium carbonate weight is added via a peristaltic pump tothe calcium carbonate slurry. Following the complete addition of acids,the slurry is stirred for an additional 5 minutes by using a propellertype mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 5 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas:

Sample V2 V10 V11 % H₃PO₄ 10 10 10 % citric acid  5  5  5 Time ofaddition Pre- simulta- t = 15% of citric acid Addition neously relativeto the total time of H₃PO₄ addition SSA of 46   50.1   48.3 product/m²/gd₅₀/μm    9.43    6.27    5.16

Example 6

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention,wherein the calcium carbonate is contacted with 10% of the at least oneacid having a pK_(a) of less than or equal to 2.5 and the time point ofadding the at least one water soluble non-polymeric organic and/orinorganic weak acid is varied.

1. Preparation of Calcium Carbonate Slurries

a. Calcium carbonate slurry V12

A calcium carbonate slurry V12 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 50% of elapsed time foraddition of H₃PO₄ (7.5 min), citric acid in an amount corresponding to2.5% by weight on calcium carbonate weight is added via a peristalticpump to the calcium carbonate slurry. Following the complete addition ofacids, the slurry is stirred for an additional 5 minutes by using apropeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

b. Calcium Carbonate Slurry V13

A calcium carbonate slurry V13 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 80% of elapsed time foraddition of H₃PO₄ (12 min), citric acid in an amount corresponding to2.5% by weight on calcium carbonate weight is added via a peristalticpump to the calcium carbonate slurry. Following the complete addition ofacids, the slurry is stirred for an additional 5 minutes by using apropeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 6 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas:

Sample V12 V13 % H₃PO₄ 10 10 % citric acid   2.5   2.5 time of addition  50%   80% of citric acid relative to the total time of H₃PO₄ additionSSA of   38.4   38.9 product/m²/g d₅₀/μm   7.6   5.9

Example 7

The following illustrative Example of the invention involves contactinga calcium carbonate according to the process of the present invention,wherein the calcium carbonate is contacted with 10% of the at least oneacid having a pK_(a) of less than or equal to 2.5 and the time point ofadding the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid is varied.

1. Preparation of Calcium Carbonate Slurries

a. Calcium Carbonate Slurry V14

A calcium carbonate slurry V14 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 80% of elapsed time foraddition of H₃PO₄ (12 min), citric acid in an amount corresponding to2.5% by weight on calcium carbonate weight is added via a peristalticpump to the calcium carbonate slurry. Following the complete addition ofacids, the slurry is stirred for an additional 5 minutes by using apropeller type mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

b. Calcium Carbonate Slurry V15

A calcium carbonate slurry V15 is prepared by adding water and 250 g ofchalk in a 5 L stainless steel reactor, such that the aqueous slurryobtained has a solids content of 10% by dry weight. The temperature ofthis slurry is thereafter brought to and maintained at 70° C. by usingof a Julabo hot-oil thermostat.

Under stirring such that an essentially laminar flow is established,H₃PO₄ in an amount corresponding to 10% by weight on calcium carbonateweight and to approximately 1×10⁻³ moles H₃PO₄ per gram calciumcarbonate is added via a peristaltic pump to the calcium carbonateslurry over a period of 15 minutes. After 15% of elapsed time foraddition of H₃PO₄ (2 min), citric acid in an amount corresponding to 5%by weight on calcium carbonate weight is added via a peristaltic pump tothe calcium carbonate slurry. Following the complete addition of acids,the slurry is stirred for an additional 5 minutes by using a propellertype mixer.

The resulting slurry is allowed to sit overnight before filtering anddrying the obtained product. The final specific BET surface area of thisdry product is measured.

The following Table 7 shows the prepared calcium carbonate slurries andthe corresponding specific BET surface areas:

Sample V14 V15 % H₃PO₄ 10 10 % citric acid  5  5 time of addition   80%  15% of citric acid relative to the total time of H₃PO₄ addition SSA of  38.9   48.3 product/m²/g d₅₀/μm   5.9    5.16

1. A surface-reacted calcium carbonate having a specific BET surfacearea of more than 50 m²/g.
 2. The surface-reacted calcium carbonateaccording to claim 1, having a specific BET surface area of more than 60m²/g.
 3. The surface-reacted calcium carbonate according to claim 1,having a specific BET surface area of more than 80 m²/g.
 4. Thesurface-reacted calcium carbonate according to claim 1, prepared by theprocess comprising the following steps: a) providing a calciumcarbonate; b) providing 5 wt.-% to 50 wt.-%, based on the weight ofcalcium carbonate, of at least one acid having a pK_(a) of less than orequal to 2.5, wherein the corresponding acid anion is capable of formingwater-insoluble calcium salts; c) providing gaseous CO₂; d) providing atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or a hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid; e) contacting said calcium carbonatewith said at least one acid having a pK_(a) of less than or equal to 2.5of step b), with said gaseous CO₂ of step c) and with said at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid of step d) to obtain surface-reacted calciumcarbonate; and f) optionally drying the surface-reacted calciumcarbonate; wherein the at least one water soluble non-polymeric organicand/or inorganic weak acid has a pK_(a) of greater than 2.5 and whereinits corresponding acid anion is capable of forming water-insolublecalcium salts.
 5. The surface-reacted calcium carbonate according toclaim 4, wherein the at least one acid having a pK_(a) of less than orequal to 2.5 is selected from the group consisting of strong acidshaving a pK_(a) value of less than or equal to zero at 25° C., andmedium-strong acids having a pK_(a) value of between zero and 2.5 at 25°C.
 6. The surface-reacted calcium carbonate according to claim 4,wherein the at least one acid having a pK_(a) of less than or equal to2.5 is H₃PO₄, oxalic acid or a mixture thereof.
 7. The surface-reactedcalcium carbonate according to claim 4, wherein the at least one acidhaving a pK_(a) of less than or equal to 2.5 is H₃PO₄.
 8. Thesurface-reacted calcium carbonate according to claim 4, wherein the atleast one acid having a pK_(a) of less than or equal to 2.5 is H₃PO₄ ina concentration of 20% to 40% (v/v).
 9. The surface-reacted calciumcarbonate according to claim 4, wherein the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid is selected from the group consisting of boric acid, citricacid, sodium dihydrogen citrate, potassium dihydrogen citrate, disodiumhydrogen citrate, dipotassium hydrogen citrate, sodium dihydrogenborate, potassium dihydrogen borate, disodium hydrogen borate,dipotassium hydrogen borate and mixtures thereof.
 10. Thesurface-reacted calcium carbonate according to claim 4, wherein the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid is citric acid.
 11. Thesurface-reacted calcium carbonate according to claim 4, wherein the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid is added in an amount in the range of0.1 wt.-% and 20 wt.-%, based on the weight of calcium carbonate. 12.The surface-reacted calcium carbonate according to claim 4, wherein theat least one water soluble non-polymeric organic and/or inorganic weakacid and/or hydrogen salt of said at least one water solublenon-polymeric organic and/or inorganic weak acid is added in an amountin the range of 0.1 wt.-% and 15 wt.-%, based on the weight of calciumcarbonate.
 13. The surface-reacted calcium carbonate according to claim4, wherein the at least one water soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid is added in anamount in the range of 0.1 wt.-% and 10 wt.-%, based on the weight ofcalcium carbonate.
 14. The surface-reacted calcium carbonate accordingto claim 4, wherein the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid is addedin an amount in the range of 0.1 wt.-% and 5 wt.-%, based on the weightof calcium carbonate.
 15. The surface-reacted calcium carbonateaccording to claim 4, wherein the at least one acid having a pK_(a) ofless than or equal to 2.5 is provided is provided in the form of: (i) atleast one H₃O⁺-ion provider having a pKa of less than or equal to 2.5,wherein the corresponding acid anion is capable of forming awater-soluble calcium salt, and (ii) at least one anion, provided in theform of a water-soluble salt or hydrogen salt, wherein the correspondingacid of this at least one anion has a pK_(a) of less than or equal to2.5 and wherein said anion is capable of forming water-insoluble calciumsalts.
 16. The surface-reacted calcium carbonate according to claim 4,wherein the calcium carbonate is further contacted with a compoundselected from the group consisting of a sodium silicate, a potassiumsilicate a lithium silicate, an aluminium silicate, a synthetic silica,a calcium silicate, an aluminium hydroxide, a sodium aluminate, apotassium aluminate and mixtures thereof.
 17. The surface-reactedcalcium carbonate according to claim 4, wherein the at least one acidhaving a pK_(a) of less than or equal to 2.5 is H₃PO₄, and the at leastone water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid is citric acid.
 18. Tissue paper,plastic, paint, a controlled release or a water treatment agentcomprising surface-reacted calcium carbonate prepared by the processcomprising the following steps: a) providing a calcium carbonate; b)providing 5 wt.-% to 50 wt.-%, based on the weight of calcium carbonate,of at least one acid having a pK_(a) of less than or equal to 2.5,wherein the corresponding acid anion is capable of formingwater-insoluble calcium salts; c) providing gaseous CO₂; d) providing atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or a hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid; e) contacting said calcium carbonatewith said at least one acid having a pK_(a) of less than or equal to 2.5of step b), with said gaseous CO₂ of step c) and with said at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid of step d) to obtain surface-reacted calciumcarbonate; and f) optionally drying the surface-reacted calciumcarbonate; wherein the at least one water soluble non-polymeric organicand/or inorganic weak acid has a pK_(a) of greater than 2.5 and whereinits corresponding acid anion is capable of forming water-insolublecalcium salts.
 19. The tissue paper, plastic, paint, controlled releaseor a water treatment agent according to claim 18, wherein the at leastone acid having a pK_(a) of less than or equal to 2.5 is selected fromthe group consisting of strong acids having a pK_(a) value of less thanor equal to zero at 25° C., and medium-strong acids having a pK_(a)value of between zero and 2.5 at 25° C.
 20. The tissue paper, plastic,paint, controlled release or a water treatment agent according to claim18, wherein the at least one acid having a pK_(a) of less than or equalto 2.5 is H₃PO₄, oxalic acid or a mixture thereof.
 21. The tissue paper,plastic, paint, controlled release or a water treatment agent accordingto claim 18, wherein the at least one acid having a pK_(a) of less thanor equal to 2.5 is H₃PO₄.
 22. The tissue paper, plastic, paint,controlled release or a water treatment agent according to claim 18,wherein the at least one acid having a pK_(a) of less than or equal to2.5 is H₃PO₄ in a concentration of 20% to 40% (v/v).
 23. The tissuepaper, plastic, paint, controlled release or a water treatment agentaccording to claim 18, wherein the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid is selected from the group consisting of boric acid, citricacid, sodium dihydrogen citrate, potassium dihydrogen citrate, disodiumhydrogen citrate, dipotassium hydrogen citrate, sodium dihydrogenborate, potassium dihydrogen borate, disodium hydrogen borate,dipotassium hydrogen borate and mixtures thereof.
 24. The tissue paper,plastic, paint, controlled release or a water treatment agent accordingto claim 18, wherein the at least one water soluble non-polymericorganic and/or inorganic weak acid and/or hydrogen salt of said at leastone water soluble non-polymeric organic and/or inorganic weak acid iscitric acid.
 25. The tissue paper, plastic, paint, controlled release ora water treatment agent according to claim 18, wherein the at least onewater soluble non-polymeric organic and/or inorganic weak acid and/orhydrogen salt of said at least one water soluble non-polymeric organicand/or inorganic weak acid is added in an amount in the range of 0.1wt.-% and 20 wt.-%, based on the weight of calcium carbonate.
 26. Thetissue paper, plastic, paint, controlled release or a water treatmentagent according to claim 18, wherein the at least one water solublenon-polymeric organic and/or inorganic weak acid and/or hydrogen salt ofsaid at least one water soluble non-polymeric organic and/or inorganicweak acid is added in an amount in the range of 0.1 wt.-% and 15 wt.-%,based on the weight of calcium carbonate.
 27. The tissue paper, plastic,paint, controlled release or a water treatment agent according to claim18, wherein the at least one ater soluble non-polymeric organic and/orinorganic weak acid and/or hydrogen salt of said at least one watersoluble non-polymeric organic and/or inorganic weak acid is added in anamount in the range of 0.1 wt.-% and 10 wt.-%, based on the weight ofcalcium carbonate.
 28. The tissue paper, plastic, paint, controlledrelease or a water treatment agent according to claim 18, wherein the atleast one water soluble non-polymeric organic and/or inorganic weak acidand/or hydrogen salt of said at least one water soluble non-polymericorganic and/or inorganic weak acid is added in an amount in the range of0.1 wt.-% and 5 wt.-%, based on the weight of calcium carbonate.
 29. Thetissue paper, plastic, paint, controlled release or a water treatmentagent according to claim 18, wherein the at least one acid having apK_(a) of less than or equal to 2.5 is provided is provided in the formof: (i) at least one H₃O⁺-ion provider having a pKa of less than orequal to 2.5, wherein the corresponding acid anion is capable of forminga water-soluble calcium salt, and (ii) at least one anion, provided inthe form of a water-soluble salt or hydrogen salt, wherein thecorresponding acid of this at least one anion has a pK_(a) of less thanor equal to 2.5 and wherein said anion is capable of formingwater-insoluble calcium salts.
 30. The tissue paper, plastic, paint,controlled release or a water treatment agent according to claim 18,wherein the at least one acid having a pK_(a) of less than or equal to2.5 is H₃PO₄, and the at least one water soluble non-polymeric organicand/or inorganic weak acid and/or hydrogen salt of said at least onewater soluble non-polymeric organic and/or inorganic weak acid is citricacid.
 31. The tissue paper, plastic, paint, controlled release or awater treatment agent according to claim 18, wherein the surface reactedcalcium carbonate has a BET specific surface area of at least 20 m²/g.32. The tissue paper, plastic, paint, controlled release or a watertreatment agent according to claim 18, wherein the surface reactedcalcium carbonate has a BET specific surface area of at least 30 m²/g.33. The tissue paper, plastic, paint, controlled release or a watertreatment agent according to claim 18, wherein the surface reactedcalcium carbonate has a BET specific surface area of at least 40 m²/g.34. The tissue paper, plastic, paint, controlled release or a watertreatment agent according to claim 18, wherein the surface reactedcalcium carbonate has a BET specific surface area of at least 60 m²/g.35. The tissue paper, plastic, paint, controlled release or a watertreatment agent according to claim 18, wherein the surface reactedcalcium carbonate has a BET specific surface area of at least 70 m²/g.36. The tissue paper, plastic, paint, controlled release or a watertreatment agent according to claim 18, wherein the surface reactedcalcium carbonate has a BET specific surface area of at least 80 m²/g.